1. Field of the Invention
The present invention relates to radio frequency communication, and specifically to the use of radio frequency identification (RFID) tags in conjunction with one or more radio frequency sensors to determine the position of the tag or an asset to which the tag is attached within three dimensional space.
2. Description of the Related Technology
The field of RFID (Radio Frequency Identification) is growing rapidly. Applications of RFID technology are wide ranging and include counting objects as they pass near to a sensor, uniquely identifying a specific tag (hereinafter xe2x80x9ctransponderxe2x80x9d) and associated asset, and placing data within the RFID transponder for later recovery. The process of xe2x80x9creadingxe2x80x9d and communicating with an RFID transponder generally comprises bringing the transponder in proximity to an RFID sensor which emanates a radio frequency wake-up field having a limited range. The RFID transponder detects the presence of the wakeup field of the sensor, and subsequently various forms or protocols of handshake occur between the transponder and the sensor in order to exchange data. All of this communication between the transponder and the sensor is performed using radio frequency carriers of some kind. When multiple transponders are involved, anti-clash protocols of the type well understood in the data processing arts are employed in order to multiplex or provide multiple access to the sensor by the multiple transponders.
The main advantages of an RFID sensor and transponder system over other forms of ID tagging include (i) the orientation of the transponder with respect to the sensor is not critical for a correct read of the transponder information; (ii) communication can occur within comparatively harsh operating environments including dirt, grease, opaque gasses, etc.; and (iii) the communication range between the sensor and transponder can be significant (in excess of 100 feet in certain cases) even when the RF frequencies used are within the power limitations of Federal Communications Commission (FCC) rules concerning unlicensed transmitters. Accordingly, RFID technology is useful for several applications, especially those relating to security and asset management.
For example, in applications where enhanced security is desired, RFID systems using electromagnetic energy with very low frequency are attractive since the low frequency energy tends to suffer low losses from shielding materials such as metal boxes, aluminum foil, and the like. Those who would surreptitiously remove the tagged assets from a building usually try to use such shielding techniques. However, these low frequencies typically require large antennas within the transponder in order to achieve reasonable levels of RF coupling between the sensor and the transponder. It is impractical to place large wire antennas within small transponders; accordingly, comparatively small magnetic loop antennas are the coupling methods of choice for such small transponders. These magnetic loop antennas exhibit a serious drawback, however, in that they have a characteristic xe2x80x9cfigure-8xe2x80x9d sensitivity pattern and, in certain positions and/or orientations, can reject or otherwise not detect the fields generated from the sensor. Stated differently, the magnetic loop antenna of the transponder can only receive energy from the sensor antenna coils only when the orientation of the sensor and transponder coils is similar. Specifically, the xe2x80x9crejectionxe2x80x9d solid angle for a loop antenna can be thought of as a band of a certain solid angle measured from the center and oriented 360 degrees around the circumference of the loop (see FIG. 1). When such rejection occurs, the transponder may be well within the sensor""s intended wake up field, but fails to detect the sensor""s emissions, and therefore also fails to communicate therewith. A related problem is when the position and/or orientation of the transponder within the field is varied, thereby taking the sensor(s) out of the xe2x80x9cfigure-8xe2x80x9d pattern of the transponder antenna, and interrupting communication between the transponder and sensor.
Additionally, many existing RFID transponder/sensor systems do not have the ability to locate the transponder in spatial space. Those which do have this ability suffer from significant drawbacks since none of them will function using the low frequency signals needed to pass through foil and other shielding. The added capability of spatial positioning, however, allows the sensor to gather more information about the transponder, i.e., its relative location in space with respect to the sensor or some other reference point. This capability provides a very significant advantage over other asset management systems (RFID or otherwise) which can not determine the position of the assets.
Furthermore, existing RFID systems in which the transponder includes a motion sensor or other device which activates or otherwise permits the waking up of the transponder do not have provision for the transponder to communicate with the system sensor (reader) during periods when the transponder is not in motion, such as during installation or maintenance. Accordingly, such prior art transponders must be physically moved or agitated during these periods in order to enable the transponder to communicate with the sensor. This approach is cumbersome and inefficient.
Based on the foregoing, an improved apparatus and method for spatially locating an RFID transponder having a magnetic loop antenna within one or more sensor fields is needed. Furthermore, an improved apparatus and method for maintaining effectively constant and uninterrupted communication with the aforementioned RFID transponder regardless of physical position or orientation is needed. Lastly, an improved apparatus for interrogating and communicating with the RFID transponder when the transponder is not in motion or otherwise dormant is needed.
The foregoing needs are addressed by the invention disclosed herein.
In a first aspect of the invention, an improved system for and method of determining the position of one or more radio frequency identification (RFID) transponders with respect to one or more sensors is disclosed. In a first embodiment, the system comprises a plurality of stationary sensor arrays located within certain physical areas. Each sensor array comprises a plurality of antenna coils arranged in unique physical orientations with respect to each other and capable of transmitting radio frequency signals of differing phase. The RFID transponder includes a magnetic loop antenna which receives the plurality of signals generated by the antenna coils, and compares the phase of at least two of the received signals in order to determine the relative position of the transponder(s) with respect to the sensors.
In a second aspect of the invention, an improved system for and method of maintaining constant communication between one or more RFID transponders and their associated sensor(s) using a multi-message protocol is disclosed. In one embodiment, the system comprises sensor arrays having a plurality of antenna coils in a predetermined physical relationship which emit two direction finding mode (DFM) signals in succession. For the emission of the first DFM signal, each of the plurality of antenna coils is energized so as to emit a signal in its given orientation. For the emission of the second DFM signal, one of the plurality of coils is turned off such that no radio frequency signal is emitted from that coil. The spatial relationship of the transponder and individual antenna coils precludes all of the signals from each sensor array from being rejected by the transponder during the emission of both the first and second DFM signals. In this fashion, the transponder coil can be kept in constant communication with the sensor, regardless of its orientation with respect to the sensors. This feature effectively eliminates the communication problems associated with the typical xe2x80x9cfigure-8xe2x80x9d pattern associated with the transponder""s antenna coil.
In a third aspect of the invention, an improved method of determining the location of the transponder with respect to two or more sensor arrays through elimination of sensor rejection is disclosed. In one embodiment, the method comprises positioning the transponder with an internal antenna coil within the field generated by the coils of the individual sensor arrays, transmitting a first signal from each of the antenna coils of the two or more arrays, transmitting a second signal from a subset of the antenna coils of the same arrays (i.e., with one or more coils turned off), and determining the position of the transponder relative to the two or more sensor arrays based on the intensity of the first and second signals received by the antenna coil of the transponder.
In a fourth aspect of the invention, a system for and method of transmitting data between a sensor having a transmit coil and a RFID transponder having a receiving coil is disclosed. In one embodiment, the system comprises a hand-held sensor probe or wand which emits a highly intense and localized wake-up field at a predetermined frequency. This field is sensed by the receiving coil of the transponder, and its physical parameters (such as intensity and/or frequency) compared to predetermined values present within the transponder. If the sensed parameters of the wake-up field meet certain predetermined criteria, the transponder generates an internal wake-up signal, and begins communicating with the sensor. This system and method are particularly useful when using transponders having an internal motion detector, thereby allowing communication with the dormant (e.g., motionless) transponder without the need to physically move the transponder.